Several methods have been proposed for improving the weldability of zinc-base galvanized sheet steel. For example, it has been proposed, as disclosed in Japanese Patent Application Kokai (Laid-open) No. 55-110,783, to form a film of oxides such as Al.sub.2 O.sub.3 on the surface of galvanized sheet steel, thereby, taking advantage of the high melting point and high electric resistance of the oxides, to improve weldability and at the same time to prevent the contact of the electrode tip with the plating metal, thus preventing the melt loss of the tip and prolonging the life thereof.
Further, it has been proposed, as disclosed in Japanese Patent Application Kokai (Laid-open) No. 59-104,463, to form an oxide film of a ZnO/Zn ratio of 0.1-0.70 on the surface of galvanized sheet steel by heat treatment, thereby to improve weldability in a similar manner to above.
However, even these methods hardly give a satisfactory result on an industrial scale, and the improvement of weldability in galvanized sheet steel is eagerly desired.
As to the methods for improving the press-formability of zinc-base galvanized sheet steel, it has been disclosed to form a hard film on zinc-base galvanized sheet steel thereby to prevent galling between the plating and the die and improve lubricity in press working by, for example, a method of applying an electrolytic chromate treatment to the galvanized sheet steel surface to form an oxide film of Cr.sub.2 O.sub.3 as described in Japanese Patent Application Kokai (Laid-open) No. 62-185,883 and a method of applying an iron-zinc alloy plating as described in Japanese Patent Application Kokai (Laid-open) No. 62-192,597.
Further, it has been disclosed, as described in Japanese Patent Application Kokai (Laid-open) No. H-1-136,952, to coat or cover such organic substances as organic lubricating film, lubricating oil, etc. on the galvanized sheet steel surface to improve its press-formability.
However, galvanized sheet steel products obtained by these methods are unsatisfactory for use in the automotive industry in the following points.
Galvanized sheet steel is employed by users in the automotive industry through a process comprising, in outline, the step of washing the sheet steel with oil, the pressing step, the degreasing step, the phosphating step, and the painting step. In the case of electrolytic chromate-treated sheet steel, a phosphate film fails to be formed in the phosphating treatment. In the case of sheet steel coated with lubricating oil or lubricating film, a satisfactory lubricating property is not exhibited since the coated materials fall off in the washing step. Further, extra load is put on the degreasing step precedent to the phosphating treatment, resulting in a higher cost. In the case of zinc-base galvanized sheet steel to which iron-zinc alloy flash plating has been applied, on the other hand, the sheet is of a higher cost as compared with those obtained by electrolytic chromate treatment.